1. Field of the Invention
The present invention relates to a mobile communication system in which a wireless link control is performed by processing an acknowledged mode wireless link control protocol.
2. Description of the Related Art
A prior art mobile communication system in which an acknowledged mode wireless link control protocol is processed will be described with reference to FIG. 11 showing a basic construction thereof. In this specification, the mobile communication system using CDMA (Code Division Multiple Access) system will be described. As shown in FIG. 11, the CDMA mobile communication system is constructed with a plurality of mobile stations 1, a plurality of base stations 2 each for connecting a plurality of mobile stations 1 within a wireless zone and base station controller 3 for controlling a plurality of base stations 2. Base station controller 3 and base stations 2 are connected through cable links and base station 2 and mobile stations 1 are connected through wireless links.
Base station controller 3 performs a wireless link control by processing an acknowledged mode wireless link control protocol with respect to mobile stations 1. The term “acknowledged mode” means an operation mode in which an acknowledge processing is performed by formatting a protocol data unit (PDU) on a wireless link to a signal format having acknowledge function and, when there is an error in transmitting the signal, such as signal drop, a re-transmission of the signal is performed.
Base station controller 3 includes protocol terminator 301 for performing a termination processing of the acknowledged mode wireless link control protocol, failure detector 302 for performing a predetermined failure detection processing related to the wireless link control in protocol terminator 301 and call controller 303 for controlling call connection/call disconnection.
In the case of the CDMA mobile communication system, mobile station 1 is capable of simultaneously having a plurality of wireless transmission links to a plurality of base stations 2 covered by base station controller 3. In the CDMA mobile communication system, when a wireless transmission quality in a certain one of the wireless transmission links is degraded, a processing (soft hand-over) for selecting another of the wireless transmission links having acceptable transmission quality and establishing a connection between the mobile station and the base station through the another wireless link is performed. With this scheme, the call connection service for always providing an optimal wireless transmission link is performed.
FIG. 12 shows a usual re-transmission sequence of an arbitrary PDU in a case where, during an original transmission (a1, number of transmissions=1) of PDU from base station controller 3 through base station 2 to mobile station 1, the PDU is dropped in a wireless link thereof. In such case, a re-transmission (a4, number of re-transmissions=1, number of transmissions=2) of the PDU is performed upon a reception of a re-transmission request (a2, negative acknowledge) from mobile station 1 by PDU base station controller 3 or a time-out (a3) of a re-transmission timer in base station controller 3.
Base station controller 3 recognizes the state of the PDU (transmission state) from a start of the 1st transmission (a1) to a start of the re-transmission (a4) as a “new transmission state” and the state subsequent to the start of the re-transmission (a4) as a “re-transmission state”. It is assumed thereafter that, due to transmission error of PDU, the re-transmission is repeated and the PDU transmission to mobile station 1 is completed by n-th re-transmission a5 (number of re-transmissions=n, number of transmissions=(n+1)). In response thereto, mobile station 1 transmits a PDU reception acknowledge (positive acknowledge) a6 to base station controller 3. In response to the acknowledge a6, base station controller 3 confirms a success of the transmission of PDU to mobile station 1 and decides that “re-transmission state” is completed. Base station controller 3 recognizes a state after the receiving time of the acknowledge a6 as a “acknowledged state”. In summary, an arbitrary PDU in base station controller 3 can be in “new transmission state”, “re-transmission state” or “acknowledged state”.
When the wireless transmission state of the wireless link between protocol terminator 301 of base station controller 3 and mobile station 1 is unacceptable, the transmission error such as signal drop may occur. In general, when there is a PDU undelivered due to transmission error in the acknowledged mode wireless link control protocol, the re-transmission processing is executed repeatedly until a delivery thereof is confirmed. When the re-transmission is repeated, the call disconnection processing is performed provided that the delivery of PDU can not be confirmed even when the number of re-transmissions reaches the maximum number predetermined in the system.
However, in a practical use of the system, there is mobile station 1 whose wireless transmission state is not acceptable in a wireless transmission link to any base station 2, depending upon a positional relation between a wireless service area (wireless zone or cell) and mobile station 1. For such call, there is a state where, although re-transmission of a PDU occurs steadily in the acknowledged mode wireless link control between base station 2 and mobile station 1, the number of re-transmissions of the PDU never reaches the above mentioned maximum number. Since, in such state, the maximum number of re-transmissions is not reached though the end-to-end through-put is kept degraded (degradation of service quality) due to steadily occurring re-transmission, the call is not disconnected and the call connection is maintained.
Since the re-transmission processing in the acknowledged mode wireless link control protocol is frequently performed for such call which continuously exists regardless of lowered transmission quality thereof in the wireless zone due to steadily occurring re-transmission, there is a problem that through-put related to data of an upper layer is kept low. In such state, service quality expected by an end user or to be provided by the system is degraded. Further, there is a problem that an amount of time charge charged on the end user who utilizes such call is increased since, when the end user transmits a constant amount of data, a required communication time or an amount of communication data is increased for the reasons mentioned above.
FIG. 13 shows a case where the wireless transmission quality of the wireless link is acceptable and a case where the quality is degraded due to steadily occurring re-transmission. In the wireless link between mobile station 1 and base station 2, a PDU, which is a transmission signal, is transmitted from base station 2 to mobile station 1. PDUs are distinguished from each other by attaching different numbers thereto. Incidentally, acknowledge signals from mobile station 1 are not shown. An upper portion of FIG. 13 shows a transmission sequence through a wireless link having acceptable transmission quality and PDUs 1 to 5 are delivered without re-transmission caused by transmission error.
A lower portion of FIG. 13 shows a transmission sequence when the transmission quality of the wireless link is lowered due to steadily occurring re-transmissions. Describing PDU1, for example, an initial transmission (original) is failed and a re-transmission (retry 1) is executed. Two re-transmissions (retry 1 and retry 2) are failed and the delivery of PDU1 is achieved by a third re-transmission (retry 3). Since the number of failed re-transmissions is smaller than the maximum number of re-transmissions (larger than 2 in this case), the call is not disconnected.
As is clear from FIG. 13, a time required to deliver a unit amount of data when the wireless transmission quality is degraded due to steady re-transmission becomes longer than that when the wireless transmission quality is high. Since the end-to-end data is carried on a payload portion of the PDU, the throughput is lowered in proportion to frequency of occurrence of re-transmission.